A bioformulation of oyster shell, neem, sida, and citronella extracts boosts cocoa (Theobroma cacao L.) seedling development and resistance to black pod disease caused by Phytophthora megakarya

Abstract

Phytophthora megakarya is the major destroyer of cocoa production in Central and West Africa. The present study evaluated the efficacy of a bioformulation called Optimized Bio Formulation Prime (OBF') containing oyster shell extract and plant extracts (neem seed, sida weed and citronella) with tween80, on cocoa seedling production. The formulation was tested for stability and activity and it appears that it is stable at 25 °C and reduces the growth of Phytophthora megakarya by 55 % when used at 1 µL/mL. OBF' exerts its fungicidal activity by inhibiting protein synthesis and ATPase/H⁺ pump activity of Phytophthora megakarya. The GC-MS analysis of OBF' showed the presence of compounds among which citral and geraniol which have antimicrobial and insecticidal activity. After two weeks, the length and number of secondary roots of the germinated seeds had significantly increased. In addition, the use of OBF' in the in planta assay significantly increased leaf surface area. Detached leaf analysis of these seedlings showed a significant reduction in disease severity index of about 79 % compared to untreated plants with OBF'. It was found that OBF' induce PR proteins such as β-1,3-glucanases and peroxidases. A significant increase in polyphenol and polyphenol oxidase activities was also observed in OBF' treated plants. This work demonstrated the stability at 25°C and the efficacy of the OBF' formulation in the management of cocoa black pod disease and, subsequently, increasing the robustness of cocoa plant seedlings against Phytophthora megakarya by priming them with increased defense markers. OBF’ may help to protect cocoa seedlings production.